Devices are known that may be implanted within a patient's body for monitoring one or more physiological conditions and/or to provide therapeutic functions. For example, sensors or transducers may be located deep within the body for monitoring a variety of properties, such as temperature, pressure, strain, fluid flow, chemical properties, electrical properties, magnetic properties, and the like. In addition, devices may be implanted that perform one or more therapeutic functions, such as drug delivery, defibrillation, electrical stimulation, and the like.
Often it is desirable to communicate with such devices once they are implanted within a patient using an external controller, for example, to obtain data, and/or to activate or otherwise control the implant. An implant may include wire leads from the implant to an exterior surface of the patient, thereby allowing an external controller or other device to be directly coupled to the implant. Alternatively, the implant may be remotely controlled, e.g., using an external induction device. For example, an external radio frequency (RF) transmitter may be used to communicate with the implant. RF energy, however, may only penetrate a few millimeters into a body, because of the body's dielectric nature, and therefore may not be able to communicate effectively with an implant that is located deep within the body. In addition, although an RF transmitter may be able to induce a current within an implant, the implant's receiving antenna, generally a low impedance coil, may generate a voltage that is too low to provide a reliable switching mechanism.
In a further alternative, electromagnetic energy may be used to control an implant, since a body generally does not attenuate magnetic fields. The presence of external magnetic fields encountered by the patient during normal activity, however, may expose the patient to the risk of false positives, i.e., accidental activation or deactivation of the implant. Furthermore, external electromagnetic systems may be cumbersome and may not be able to effectively transfer coded information to an implant.
Notably, implantable biosensors that measure pressure deep within anatomical structures such as blood vessels or the brain, can only communicate the absolute pressure associated with the immediate anatomical environment. These devices are not capable of communicating gauge pressure because they are confined and sealed away from the ambient pressure external the body. In most cases, it is gauge pressure and not absolute pressure that is sought to be known, since the body regulates its activities based on the ambient pressure. Gauge pressure may be determined by correlating the absolute pressure with the ambient pressure. For example, Miesel et al. (U.S. Pat. No. 6,248,080), which isincorporated herein by reference, uses a barometer to determine gauge pressure based on a correlation of absolute pressure and ambient pressure. The Miesel system, however, requires a barometer to determine the ambient pressure.